The gripes over Sonic Adventure 2 compared to SA1 are really outlandish. It's a high bunch of malarchy.
Sonic Adventure 1 was a complete disgrace to the Sonic games and series in general. The adventure part was highly stupid, the story was very dull and boring, and Big the Cat was just filler material. Amy Rose should have never gone playable. The actual levels weren't that bad, but far too linear and theere wasn't enough of a selection for pure Sonic action.
SA2 is a little different, but still quite bad. I remember it being in development and the Sonic levels were supposed to have twice the amount they had, but Sonic Team Hawaii/USA wanted it to be split even. The level design is a bit better than SA1, not as linear, and the framerate was stuck at a smooth 60 fps most of the time. Tails should have been himself and not in some stupid half mech thing. Knuckles and Rouge the Bat was just a disgrace and became quite a chore finding those darned emeralds.
The voice acting for Sonic was quite bad, it should have never been Urkel. That was the worst choice anyone could have made.
Other than that, so far the Sonic games in 3d so far have been a mild success but dismally failed concept at bringing the old 2d Sonic games to 3d.
I never realised that!